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I have a question! Mars' moon...
Can anyone answer?
Does Mars' moon always point towards the planet? (like our moon always has the same face to Earth, because each body has equal and opposite rotation "due to tidal forces") also, if I can have another, is there any hard data on these tidal forces, and how long it took for them to create the equal&opposite rotation effect? cheers D |
#2
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I have a question! Mars' moon...
Diesel wrote:
Does Mars' moon always point towards the planet? (like our moon always has the same face to Earth, because each body has equal and opposite rotation "due to tidal forces") Both of them do, yes. And both of them have their long axes pointing towards Mars' center. (Phobos and Deimos are notably aspherical.) And it's not so much that Earth and the Moon have "equal and opposite rotation". Tidal effects tend to pull on irregularities in such a way as to cause most of the mass to be concentrated in the direction of the body pulling on it. (The Moon's center of mass is slightly displaced from its geographical center -- and that center of mass points Earthward as seen from the geographical center.) The effect becomes more pronounced with bodies close to one another in space or in mass. also, if I can have another, is there any hard data on these tidal forces, and how long it took for them to create the equal&opposite rotation effect? Eep. The most I've ever seen on this subject is that it takes billions of years, but exactly how long? I don't know. It is significant to note that the vast majority of moons in the Solar System are tidally locked to their primaries, though (and Pluto and Charon are even tidally locked to one another). -- -- With Best Regards, Matthew Funke ) |
#3
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I have a question! Mars' moon...
"Matthew F Funke" schrieb im Newsbeitrag ... Diesel wrote: Eep. The most I've ever seen on this subject is that it takes billions of years, but exactly how long? I don't know. It is significant to note that the vast majority of moons in the Solar System are tidally locked to their primaries, though (and Pluto and Charon are even tidally locked to one another). But since one could view planets as the suns "moons", why are they not tide locked with the sun? Lots of Greetings! Volker |
#4
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I have a question! Mars' moon...
Matthew F Funke replied to Volker Hetzer:
why are [planets] not tide locked with the sun? [I disliked the wording of Volker's question so much that I felt compelled to modify it. --jr] Well, Mercury is locked in a sort of resonance with the Sun -- kind of a simple tidal locking. As far as the rest is concerned, remember that tidal locking becomes much more pronounced as the bodies in question get closer to each other (tides go down with the *cube* of distance, not just the *square* as gravity does) and as they get close to each other in mass. The planets are *much* smaller than the Sun. Similarity of mass can't be a factor. Gravity-gradient stabilization has been used on many low Earth orbit satellites, such as some of the US Navy Transit satellites in the 1960s, the GEOS satellites in the 60s and 70s, and Geosat in 1985. Consider the Sun and the Earth versus the Earth and the Moon. The Sun is about 330,000 times as massive as the Earth, but the Earth is only about 81 times as massive as the Moon. The Earth and the Moon are *much* closer to each other in terms of mass than the Sun is to the Earth, which contributes to the Moon's faster tidal locking. (Note that this point cannot be taken strictly at face value, since proximity in mass is only one of the primary contributors to tidal locking... but it might give you some insight as to what's going on.) Mars is almost 60 million times the mass of Phobos, and more than 350 million times the mass of Deimos. I suspect that the idea is one you came up with, yourself. Not finding any information contradicting it, you gradually assumed that it was correct, because it seemed to fit the info you had. Yes? No? Maybe? -- Jeff, in Minneapolis .. |
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I have a question! Mars' moon...
Jeff Root wrote:
Matthew F Funke replied to Volker Hetzer: Well, Mercury is locked in a sort of resonance with the Sun -- kind of a simple tidal locking. As far as the rest is concerned, remember that tidal locking becomes much more pronounced as the bodies in question get closer to each other (tides go down with the *cube* of distance, not just the *square* as gravity does) and as they get close to each other in mass. The planets are *much* smaller than the Sun. Similarity of mass can't be a factor. Gravity-gradient stabilization has been used on many low Earth orbit satellites, such as some of the US Navy Transit satellites in the 1960s, the GEOS satellites in the 60s and 70s, and Geosat in 1985. As I'm sitting here thinking about it, your statement makes sense to me. After all, tide locking has more to do with tidal torque than anything, and the similarity of mass of the bodies doesn't seem to factor into that at all. What matters is the force of the gravity and the drag (or pull) that force creates on tides raised. This seems borne out by the Earth-Moon system. Gravity is a symmetrical force, after all, but the Moon is much smaller, so the Earth would be more effective at tidally locking the Moon. Consider the Sun and the Earth versus the Earth and the Moon. The Sun is about 330,000 times as massive as the Earth, but the Earth is only about 81 times as massive as the Moon. The Earth and the Moon are *much* closer to each other in terms of mass than the Sun is to the Earth, which contributes to the Moon's faster tidal locking. (Note that this point cannot be taken strictly at face value, since proximity in mass is only one of the primary contributors to tidal locking... but it might give you some insight as to what's going on.) Mars is almost 60 million times the mass of Phobos, and more than 350 million times the mass of Deimos. I suspect that the idea is one you came up with, yourself. Not finding any information contradicting it, you gradually assumed that it was correct, because it seemed to fit the info you had. Yes? No? Maybe? I doubt I came up with it myself, since it seems so bizarre. But I can't for the life of me nail down where I first heard it. Perhaps I had misheard something about the similarity in mass of the Earth-Moon system way back when in the context of tidal locking or something, and somewhere my brain assumed it was relevant, not really taking the time to think out why it might or might not be so. Besides, it's not like me to invent tidal theories as I go... a casual look at trying to figure out how the Moon's tidal influence relates to oceanic tides convinced me a long time ago that there was more to this than met the eye. In any case, thanks for forcing me to re-examine things and bringing me to task. I apologize for not thinking this through. I've re-examined (and, more importantly, researched) the statements I made about tides, and still stick by distance as an important factor (since tide strength is inversely proportional to the cube of distance). Thus, the effect of the Moon on tides is approximately [(7.349e22 kg)/(384403 km)^3]/[(1.989e30 kg)/(149597890 km)^3] = 2.18 times the effect of the Sun's effect on tides. (Note that even though I was fast and loose with units, they cancel out.) Thus, one can say that the Sun hasn't tidally locked the planets because the interplanetary distances are much greater than the distances between the planet and its moon(s). Mass is important, yes, but distance is much *more* important. And closeness of mass of the two bodies doesn't have squat to do with anything, and I apologize for parroting that. -- -- With Best Regards, Matthew Funke ) |
#6
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I have a question! Mars' moon...
In message , Jeff Root
writes Matthew F Funke replied to Volker Hetzer: why are [planets] not tide locked with the sun? [I disliked the wording of Volker's question so much that I felt compelled to modify it. --jr] Well, Mercury is locked in a sort of resonance with the Sun -- kind of a simple tidal locking. As far as the rest is concerned, remember that tidal locking becomes much more pronounced as the bodies in question get closer to each other (tides go down with the *cube* of distance, not just the *square* as gravity does) and as they get close to each other in mass. The planets are *much* smaller than the Sun. Similarity of mass can't be a factor. Gravity-gradient stabilization has been used on many low Earth orbit satellites, such as some of the US Navy Transit satellites in the 1960s, the GEOS satellites in the 60s and 70s, and Geosat in 1985. Isn't the important factor the gradient across the object? As you say, it's actually called gravity-gradient stabilisation when it's applied to artificial satellites (it's also more syllables than "tidal locking", so it sounds cool). -- "Forty millions of miles it was from us, more than forty millions of miles of void" |
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